In existing electro-mechanical micromachined structures, such as sensors and accelerometers, certain micromachined silicon structures are designed to move relative to the substrate or other micromachined structures in response to forces being applied in a predetermined manner, such as along a predetermined axis of the device. The movement of certain of the structures permits the generation of signals proportional to the magnitude, direction, and/or duration of the force.
Typically, the movable structures are at a different potential from other structures. For example, a movable structure might be constructed with fingers adjacent and parallel to the fingers of one or more fixed (non-moving) structures, with all of these structures in a plane above the substrate. The movable structure and the fixed structure form a capacitor, whose capacitance changes when the movable structure moves in response to the force. A micromachined structure utilizing this arrangement to detect acceleration is described in commonly assigned U.S. Pat. Nos. 5,345,824 and 5,314,572, which are incorporated herein by reference.
If a finger from the movable structure moves into contact with a finger from the fixed structure, the fingers can stick due to electrostatic attraction and even fuse together, causing the device to fail. Even where the movable structure moves into contact with another structure that is nominally at the same potential, if the two structures are not fabricated from the same material, there will be a difference in work functions that will cause an electrostatic attraction and possible device failure through stiction. Even if the materials are nominally the same, there will be small differences in work function due to small differences in composition, if they were deposited at different times during the device fabrication.
Electrostatic attraction is one of several forces responsible for stiction of contacting structures. Van der Waals and capillary forces (if water vapor is present) are two additional forces that contribute to stiction. The present invention eliminates the electrostatic force which occurs when structures not at the same potential come into contact and, therefore, makes the device more robust against stiction.